CHEMISTRY. Chapter 11 Intermolecular Forces Liquids and Solids

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1 CHEMISTRY The Central Science 8 th Edition Chapter 11 Liquids and Solids Kozet YAPSAKLI

2 States of Matter difference between states of matter is the distance between particles. In the solid and liquid states particles are closer together, we refer to them as condensed phases.

3 The States of Matter The state of a substance at a particular temperature and pressure depends on : The kinetic energy of the particles The strength of the attractions between the particles

4 Converting a gas into a liquid or solid requires the molecules to get closer to each other: cool or compress. Converting a solid into a liquid or gas requires the molecules to move further apart: heat or reduce pressure. The forces holding solids and liquids together are called intermolecular forces.

5 physical properties such as boiling and melting points, vapor pressures, and viscosities.

6 The attraction between molecules is an intermolecular force. These intermolecular forces as a group are referred to as van der Waals forces.

7 van der Waals Dipole-dipole interactions London dispersion forces Hydrogen bonding

8 Ion-Dipole Interactions An ion-dipole force exists between an ion and the partial charge on the end of a polar molecule. Ion-dipole interactions are an important force in solutions of ions.

9 Dipole-Dipole Interactions Dipole-dipole forces exist between neutral polar molecules. Molecules that have regular dipoles are attracted to each other. The positive end of one is attracted to the negative end of the other and vice-versa. These forces are only important when the molecules are close to each other.

10 Dipole-Dipole Interactions The greater the polarity of the molecules, the stronger the attractions between them. The more polar the molecule, the higher boiling point.

11 London Dispersion It is possible for two adjacent neutral molecules to affect each other. The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom). London dispersion forces, are attractions between an instantaneous dipole and an induced dipole.

12 London Dispersion These forces are present in all molecules, whether they are polar or nonpolar. The ease with which the electron distribution in a molecule is distorted is called polarizability. London dispersion forces depend on the shape of the molecule.

13 Factors Affecting London The greater the surface area available for contact, the greater the dispersion forces. The shape of the molecule affects the strength of dispersion forces: The strength of dispersion forces tends to increase with increased molecular weight. Larger atoms have larger electron clouds, which are easier to polarize.

14 Which Have a Greater Effect: Dipole-Dipole Interactions or Dispersion? If two molecules are of comparable size and shape, dipole-dipole interactions will likely be the dominating force. If one molecule is much larger than another, dispersion forces will likely determine its physical properties.

15 How Do We Explain This? The nonpolar series (SnH 4 to CH 4 ) follow the expected trend. The polar series follows the trend from H 2 Te through H 2 S, but water is quite an anomaly.

16 Hydrogen Bonding is a special type of intermolecular attraction that exists btw the hydrogen atom in a polar bond. The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong. Among the strongest of the van der Waals forces are hydrogen bonds.

17 Summarizing

18 Affect Many Physical Properties The strength of the attractions between particles can greatly affect the properties of a substance or solution.

19 Viscosity Resistance of a liquid to flow is called viscosity. Viscosity increases with stronger intermolecular forces and decreases with higher temperature.

20 Phase Changes

21 Energy Changes Heat of Fusion: Energy required to change a solid at its melting point to a liquid. Heat of Vaporization: Energy required to change a liquid at its boiling point to a gas.

22 Energy Changes Associated with Changes of State The heat added to the system at the melting and boiling points goes into pulling the molecules farther apart from each other. The temperature of the substance does not rise during the phase change.

23 Vapor Pressure At any temperature, some molecules in a liquid have enough energy to escape. As the temperature rises, the fraction of molecules that have enough energy to escape increases.

24 Vapor Pressure As more molecules escape the liquid, the pressure they exert increases. The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate.

25 Vapor Pressure The boiling point of a liquid is the temperature at which its vapor pressure equals atmospheric pressure. The normal boiling point is the temperature at which its vapor pressure is 760 torr. Liquids boil when the external pressure equals the vapor pressure.

26 Phase Diagrams Phase diagrams display the state of a substance at various pressures and temperatures and the places where equilibria exist between phases.

27 Phase Diagrams The AB line is the liquid-vapor interface. It starts at the triple point (A), the point at which all three states are in equilibrium. It ends at the critical point (B); above this critical temperature and pressure the liquid and vapor are indistinguishable from each other. Each point along this line is the boiling point

28 Phase Diagrams The AD line is the interface between liquid and solid. The melting point at each pressure can be found along this line. Below A the substance cannot exist in the liquid state. Along the AC line the solid and gas phases are in equilibrium; the sublimation point at each pressure is along this line.

29 Phase Diagram of Water Note the high critical temperature and critical pressure: These are due to the strong van der Waals forces between water molecules.

30 Phase Diagram of Carbon Dioxide Carbon dioxide cannot exist in the liquid state at pressures below 5.11 atm; CO 2 sublimes at normal pressures. The low critical temperature and critical pressure for CO 2 make supercritical CO 2 a good solvent for extracting nonpolar substances (such as caffeine).

31 Not responsible Structures of Solids Bonding in Solids